Surface flatness is a common measurement specification over a wide range of manufacturing industries. Flatness critically affects, for example, the reliability and assembly yield of electronic products, the cosmetic appearance and handling characteristics of paper products, and the mechanical fit and functionality of fabricated metal components. Non-flatness, or warpage, is a frequent problem in manufacturing, due to inadequacies in design, materials, and/or processing of components. The ability to distinguish and reject components whose non-flatness exceeds user specifications is valuable on the production line because it allows the manufacturer or user to avoid problems in later manufacturing steps, maintain product quality, and recognize processing problems early.
Shadow moiré measurement techniques have previously been applied to measure surface flatness in printed circuit boards and other electronic packaging components. Shadow moiré is an optical method for measuring relative vertical displacement of (semi-) continuous opaque surfaces. It is a full-field technique, i.e., it simultaneously acquires optical data across an entire sample. Shadow moiré is based on the geometric interference of a shadow grating projected on the sample surface and a real grating on a flat reference surface. For example, when a printed circuit board is viewed through a grating and a shadow of the grating is cast upon the surface of the printed circuit board, the shadow and the grating can interact to create a shadow moiré fringe pattern that is indicative of the warpage of the surface of the printed circuit board.
FIG. 1 illustrates an exemplary system 100 for measuring surface flatness of a sample 105 utilizing traditional shadow moiré measurement techniques. The system 100 comprises a light source 110, a grating 120 suspended above a sample 105, and a camera 115, e.g., a charge coupled device (CCD) camera, associated with a computer 125. The grating 120 is of the Ronchi type, comprising a generally planar plate of transparent material that includes multiple parallel and evenly spaced opaque lines extending across the surface of the plate. Typically, the grating 120 has a periodicity of 50 to 500 lines per inch. The center-to-center distance between the lines, the pitch (“P”) of the grating, is constant. The grating 120 is generally parallel to the sample 105. The light source 110 is a continuous or pulsed white light source of the line source type, wherein the line is parallel to the lines of the grating 120 surface.
FIG. 2 illustrates an exemplary instance 200 of the shadow moiré technique. Referring to FIGS. 1 and 2, the light source 110 illuminates the grating 120 and the sample 105 at an oblique angle of incidence. The light 111 projects a shadow 215 of the grating 120 (i.e., a shadow of the opaque lines of the grating 120, referred to herein as a “shadow grating” 215) onto the sample 105. The camera 115 captures one or more images 112 of the grating 120, the sample 105, and the shadow grating 215. The camera typically observes the image(s) 112 at an angle of 0° (normal). A stationary support structure (not shown) holds the sample 105 in place during the period in which the camera 115 captures the image(s) 112.
The overlap of the shadow grating 215 and the real grating 120 in the image(s) 112 is a periodic function of the distance between them. When the surface of the sample 105 is curved or warped, a series of dark and light fringes (moiré fringes) are produced as a result of the geometric interference pattern created between the reference grating 120 and shadow grating 215. The moiré fringes are indicative of the warpage of the sample 105. In other words, the moiré fringes correspond to contour lines of the topography of the upper surface of the sample 105. The computer 125 associated with the camera 115 comprises software that can quantify the warpage from the shadow moiré fringe pattern.
FIG. 3 is a schematic view of an exemplary shadow moiré fringe pattern 300 that could be received by the camera 115 (FIG. 1) in the above-described circumstances. The shadow moiré fringe pattern 300 comprises a series of dark and light fringes 305. In general, the greater the warpage of the sample 105, the larger the number of fringes 305. Each successive fringe represents a height change of the sample surface of W, the height per fringe. W can be calculated with the following equation:
      W    =          P                        tan          ⁢                                          ⁢          a                +                  tan          ⁢                                          ⁢          b                      ,where P is the pitch of the grating, a is the angle of incidence, and b is the angle of observation.
FIG. 4, comprising FIGS. 4a and 4b, illustrates a system 400 for measuring surface flatness of a sample 105 using the phase stepping analytical method. Phase stepping is an analytical method that has been used to improve the resolution of the shadow moiré technique and automatically identify the direction of height change from fringe to fringe across the sample 105. Without phase stepping or some equivalent technique, shadow moiré is of limited usefulness for routine automated analysis.
In conventional phase stepping, a conveyor 145 transports a sample 105 beneath a grating 120 and camera 115 (FIG. 1). Once the sample 105 is beneath the grating 120 and camera 115, the conveyor 145 stops moving the sample 105, and the camera 115 captures three or more images 112 (FIG. 1) of the unmoving sample 105 as part of each measurement. Between each successive image 112, a high precision vertical motion system (not shown) translates the sample 105 and grating 120 uniformly closer or farther away by a fixed distance or phase step. The motion system is typically expensive, especially if the sample and/or the grating is large or heavy. The motion system can physically move either the sample 105 or the grating 120, and the typical distance is on the order of 0.0025 inches. Each physical movement of the sample 105 or grating 120 is generally referred to herein as a “phase step.” FIG. 4a illustrates an exemplary system 400a for producing a phase step by vertically translating the grating 120 relative to the sample 105. FIG. 4b illustrates an exemplary system 400b for producing a phase step by vertically translating the sample 105 relative to the grating 120.
At each point on the sample 105 surface, where a point is defined as the area imaged by one camera pixel, the computer 125 (FIG. 1) measures three or more values of light intensity. The computer 125 can calculate a phase value from the intensity values, as described in U.S. Pat. No. 5,969,819 to Wang, which is hereby incorporated by reference. From the phase value, the computer 125 can calculate the relative height of all points on the sample 105 surface. The calculations assume that the phase step translation distance is precisely a simple fraction of the quantity W defined above, e.g., one-fourth of W if four images are acquired, and that the motion system that translates the grating 120 or sample 105 reproduces that step size precisely. If these conditions are not met, errors are introduced into the analysis. These precision requirements for the motion system generally make the analysis slow and expensive.
Many manufacturing industries require surface flatness measurement of continuously moving samples. For example, where materials, such as paper, are formed, woven, or extruded on a continuous basis, production flows cannot be stopped to measure samples of the materials without inducing flaws into the materials. In addition, some samples cannot readily be stopped and started because of sample characteristics, such as high weight, and/or the requirements of the remainder of the production line. Because the above-described techniques require a stationary support structure to hold the sample in place during data acquisition and/or grating/sample translation, they cannot readily be applied to measure surface flatness of continuously moving samples.
In view of the foregoing, a need exists in the art for a system and method for measuring surface flatness of continuously moving samples. In addition, a need exists for such a system and method to be efficient and cost-effective.